CS107, Lecture 5 - Stanford University · CS107, Lecture 5 More C Strings Reading: K&R (1.6, 5.5,...

Based on slides created by Marty Stepp, Cynthia Lee, Chris Gregg, Lisa Yan, Jerry Cain and others.

CS107, Lecture 5More C Strings

Reading: K&R (1.6, 5.5, Appendix B3) or Essential C section 3

Lecture Plan• Searching in Strings• Practice: Password Verification• Demo: Buffer Overflow and Valgrind• Pointers• Strings in Memory

cp -r /afs/ir/class/cs107/lecture-code/lect5 .

C StringsC strings are arrays of characters ending with a null-terminating character '\0'.

String operations such as strlen use the null-terminating character to find the end of the string.

Side note: use strlen to get the length of a string. Don’t use sizeof!

index 0 1 2 3 4 5 6 7 8 9 10 11 12 13

value 'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!' '\0'

Common string.h FunctionsFunction Description

strlen(str) returns the # of chars in a C string (before null-terminating character).

strcmp(str1, str2), strncmp(str1, str2, n)

compares two strings; returns 0 if identical, <0 if str1 comes before str2 in alphabet, >0 if str1 comes after str2 in alphabet. strncmpstops comparing after at most n characters.

strchr(str, ch)strrchr(str, ch)

character search: returns a pointer to the first occurrence of ch in str, or NULL if ch was not found in str. strrchr find the last occurrence.

strstr(haystack, needle) string search: returns a pointer to the start of the first occurrence of needle in haystack, or NULL if needle was not found in haystack.

strcpy(dst, src),strncpy(dst, src, n)

copies characters in src to dst, including null-terminating character. Assumes enough space in dst. Strings must not overlap. strncpystops after at most n chars, and does not add null-terminating char.

strcat(dst, src),strncat(dst, src, n)

concatenate src onto the end of dst. strncat stops concatenating after at most n characters. Always adds a null-terminating character.

strspn(str, accept),strcspn(str, reject)

strspn returns the length of the initial part of str which contains onlycharacters in accept. strcspn returns the length of the initial part of str which does not contain any characters in reject.

Searching For Lettersstrchr returns a pointer to the first occurrence of a character in a string, or NULL if the character is not in the string.

char daisy[6];strcpy(daisy, "Daisy");char *letterA = strchr(daisy, 'a');printf("%s\n", daisy); // Daisyprintf("%s\n", letterA); // aisy

If there are multiple occurrences of the letter, strchr returns a pointer to the first one. Use strrchr to obtain a pointer to the last occurrence.

Searching For Stringsstrstr returns a pointer to the first occurrence of the second string in the first, or NULL if it cannot be found.

char daisy[10];strcpy(daisy, "Daisy Dog");char *substr = strstr(daisy, "Dog");printf("%s\n", daisy); // Daisy Dogprintf("%s\n", substr); // Dog

If there are multiple occurrences of the string, strstr returns a pointer to the first one.

String Spansstrspn returns the length of the initial part of the first string which contains only characters in the second string.

char daisy[10];strcpy(daisy, "Daisy Dog");int spanLength = strspn(daisy, "aDeoi"); // 3

“How many places can we go in the first string before I encounter a character not in the second string?”

String Spansstrcspn (c = “complement”) returns the length of the initial part of the first string which contains only characters not in the second string.

char daisy[10];strcpy(daisy, "Daisy Dog");int spanLength = strcspn(daisy, "driso"); // 2

“How many places can we go in the first string before I encounter a character in the second string?”

C Strings As ParametersWhen we pass a string as a parameter, it is passed as a char *. We can still operate on the string the same way as with a char[]. (We’ll see why today!).

int doSomething(char *str) {char secondChar = str[1];...

// can also write this, but it is really a pointerint doSomething(char str[]) { ...

Arrays of StringsWe can make an array of strings to group multiple strings together:

char *stringArray[5]; // space to store 5 char *s

We can also use the following shorthand to initialize a string array:

char *stringArray[] = {"Hello","Hi","Hey there"

Arrays of StringsWe can access each string using bracket syntax:

printf("%s\n", stringArray[0]); // print out first string

When an array is passed as a parameter in C, C passes a pointer to the first element of the array. This is what argv is in main! This means we write the parameter type as:

void myFunction(char **stringArray) {

// equivalent to this, but it is really a double pointervoid myFunction(char *stringArray[]) {

Practice: Password VerificationWrite a function verifyPassword that accepts a candidate password and certain password criteria and returns whether the password is valid.

bool verifyPassword(char *password, char *validChars, char *badSubstrings[], int numBadSubstrings);

password is valid if it contains only letters in validChars, and does not contain any substrings in badSubstrings.

Practice: Password Verificationbool verifyPassword(char *password, char *validChars, char *badSubstrings[], int numBadSubstrings);

Example:

char *invalidSubstrings[] = { "1234" };

bool valid1 = verifyPassword("1572", "0123456789",invalidSubstrings, 1); // true

bool valid2 = verifyPassword("141234", "0123456789",invalidSubstrings, 1); // false

Practice: Password Verification

verify_password.c

Buffer Overflows• We must always ensure that memory operations we perform don’t improperly

read or write memory.• E.g. don’t copy a string into a space that is too small!• E.g. don’t ask for the string length of an uninitialized string!

• The Valgrind tool may be able to help track down memory-related issues.• See cs107.stanford.edu/resources/valgrind• We’ll talk about Valgrind more when we talk about dynamically-allocated memory.

Demo: Memory Errors

memory_errors.c

Pointers• A pointer is a variable that stores a memory address.• Because there is no pass-by-reference in C like in C++, pointers let us pass

around the address of one instance of memory, instead of making many copies.• One (8 byte) pointer can refer to any size memory location!• Pointers are also essential for allocating memory on the heap, which we will

cover later.• Pointers also let us refer to memory generically, which we will cover later.

Memory• Memory is a big array of bytes.• Each byte has a unique numeric index that is

commonly written in hexadecimal.• A pointer stores one of these memory addresses.

Address Value

0x105 '\0'

0x104 'e'

0x103 'l'

0x102 'p'

0x101 'p'

0x100 'a'

Memory• Memory is a big array of bytes.• Each byte has a unique numeric index that is

commonly written in hexadecimal.• A pointer stores one of these memory addresses.

Address Value

261 '\0'

260 'e'

259 'l'

258 'p'

257 'p'

256 'a'

Looking Back at C++How would we write a program with a function that takes in an int and modifies it? We might use pass by reference.

void myFunc(int& num) {num = 3;

int main(int argc, char *argv[]) {int x = 2;myFunc(x);printf("%d", x); // 3!...

Looking Ahead to C• All parameters in C are “pass by value.” For efficiency purposes, arrays (and

strings, by extension) passed in as parameters are converted to pointers.• This means whenever we pass something as a parameter, we pass a copy.• If we want to modify a parameter value in the function we call and have the

changes persist afterwards, we can pass the location of the value instead of the value itself. This way we make a copy of the address instead of a copy of the value.

Pointersint x = 2;

// Make a pointer that stores the address of x.// (& means "address of")int *xPtr = &x;

// Dereference the pointer to go to that address.// (* means "dereference")printf("%d", *xPtr); // prints 2

PointersA pointer is a variable that stores a memory address.

void myFunc(int *intPtr) {*intPtr = 3;

int main(int argc, char *argv[]) {int x = 2;myFunc(&x);printf("%d", x); // 3!...

mainSTACK

myFunc

intPtr

myFunc

intPtr

myFunc

intPtr

STACKAddress Value

0x1f0 2…

xmain()

Address Value…

0x1f0 2…

xmain()

Address Value…

0x1f0 2…

0x10 0x1f0…

intPtr

main()

myFunc()

Address Value…

0x1f0 2…

0x10 0x1f0…

intPtr

main()

myFunc()

Address Value…

0x1f0 3…

0x10 0x1f0…

intPtr

main()

myFunc()

Address Value…

0x1f0 3…

xmain()

Address Value…

0x1f0 3…

xmain()

Pointers Summary• If you are performing an operation with some input and do not care about any

changes to the input, pass the data type itself. This makes a copy of the data.• If you are modifying a specific instance of some value, pass the location of

what you would like to modify. This makes a copy of the data’s location.• If a function takes an address (pointer) as a parameter, it can go to that

address if it needs the actual value.

PointersWithout pointers, we would make copies.

void myFunc(int val) {val = 3;

STACKAddress Value

0x1f0 2…

xmain()

STACKAddress Value

0x1f0 2…

xmain()

STACKAddress Value

0x1f0 2…

0x10 2…

main()

myFunc()

STACKAddress Value

0x1f0 2…

0x10 2…

main()

myFunc()

STACKAddress Value

0x1f0 2…

0x10 3…

main()

myFunc()

STACKAddress Value

0x1f0 2…

xmain()

STACKAddress Value

0x1f0 2…

xmain()

Strings In Memory1. If we create a string as a char[], we can modify its characters because its memory

lives in our stack space. 2. We cannot set a char[] equal to another value, because it is not a pointer; it refers

to the block of memory reserved for the original array.3. If we pass a char[] as a parameter, set something equal to it, or perform arithmetic

with it, it’s automatically converted to a char *.4. If we create a new string with new characters as a char *, we cannot modify its

characters because its memory lives in the data segment.5. We can set a char * equal to another value, because it is a reassign-able pointer.6. Adding an offset to a C string gives us a substring that many places past the first

character.7. If we change characters in a string parameter, these changes will persist outside of the

function.

String Behavior #1: If we create a string as a char[], we can modify its characters because its memory lives in our stack space.

Character ArraysAddress Value

0x105 '\0'

0x104 'e'

0x103 'l'

0x102 'p'

0x101 'p'

0x100 'a'

When we declare an array of characters, contiguous memory is allocated on the stack to store the contents of the entire array. We can modify what is on the stack.

char str[6];strcpy(str, "apple");

String Behavior #2: We cannot set a char[] equal to another value, because it is not a pointer; it refers to the block of memory reserved for the original array.

Character ArraysAn array variable refers to an entire block of memory. We cannot reassign an existing array to be equal to a new array.char str[6];strcpy(str, "apple");char str2[8];strcpy(str2, "apple 2");

str = str2; // not allowed!

An array’s size cannot be changed once we create it; we must create another new array instead.

String Behavior #3: If we pass a char[] as a parameter, set something equal to it, or perform arithmetic with it, it’s automatically converted to a char *.

String ParametersHow do you think the parameter str is being represented?

void fun_times(char *str) {...

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");fun_times(local_str);return 0;

A. A copy of the array local_strB. A pointer containing an address to

the first element in local_str

local_str

0xa0 0xa1 0xa2 0xa3 0xa4

'r' 'i' 'c' 'e' '\0'

String ParametersHow do you think the parameter str is being represented?

void fun_times(char *str) {...

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");fun_times(local_str);return 0;

local_str

'r' 'i' 'c' 'e' '\0'

str 0xa0

char * VariablesHow do you think the local variable str is being represented?

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");char *str = local_str;...return 0;

local_str

'r' 'i' 'c' 'e' '\0'

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");char *str = local_str;...return 0;

local_str

'r' 'i' 'c' 'e' '\0'

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");char *str = local_str + 2;...return 0;

local_str

'r' 'i' 'c' 'e' '\0'

A. A copy of part of the array local_strB. A pointer containing an address to

the third element in local_str

int main(int argc, char *argv[]) {char local_str[5];strcpy(local_str, "rice");char *str = local_str + 2;...return 0;

local_str

'r' 'i' 'c' 'e' '\0'

A. A copy of part of the array local_strB. A pointer containing an address to

the third element in local_str

String ParametersAll string functions take char * parameters – they accept char[], but they are implicitly converted to char * before being passed.

• strlen(char *str)• strcmp(char *str1, char *str2)• …

• char * is still a string in all the core ways a char[] is• Access/modify characters using bracket notation• Print it out• Use string functions• But under the hood they are represented differently!

• Takeaway: We create strings as char[], pass them around as char *

String Behavior #4: If we create a new string with new characters as a char *, we cannot modify its characters because its memory lives in the data segment.

char *

There is another convenient way to create a string if we do not need to modify it later. We can create a char * and set it directly equal to a string literal.

char *myString = "Hello, world!";char *empty = "";

myString[0] = 'h'; // crashes!printf("%s", myString); // Hello, world!

char *

Address Value…

0xff0 0x10……

0x12 '\0'0x11 'i'0x10 'h'

When we declare a char pointer equal to a string literal, the characters are not stored on the stack. Instead, they are stored in a special area of memory called the “data segment”. We cannot modify memory in this segment.char *str = "hi";The pointer variable (e.g. str) refers to the address of the first character of the string in the data segment.

strSTACK

DATA SEGMENT

This applies only to creating newstrings with char *. This does notapply for making a char * that points to an existing stack string.

Memory LocationsFor each code snippet below, can we modify the characters in myStr?

char myStr[6];

Key Question: where do its characters live? Do they live in memory we own? Or the read-only data segment?

char *myStr = "Hi";

char buf[6];strcpy(buf, "Hi");char *myStr = buf;

char *otherStr = "Hi";char *myStr = otherStr;

void myFunc(char *myStr) {...

int main(int argc, char *argv[]) {char buf[6];strcpy(buf, "Hi");myFunc(buf);return 0;

Memory LocationsQ: Is there a way to check in code whether a string’s characters are modifiable?A: No. This is something you can only tell by looking at the code itself and how the string was created.

Q: So then if I am writing a string function that modifies a string, how can I tell if the string passed in is modifiable?A: You can’t! This is something you instead state as an assumption in your function documentation. If someone calls your function with a read-only string, it will crash, but that’s not your function’s fault :-)

String Behavior #5: We can set a char * equal to another value, because it is a reassign-able pointer.

char *

A char * variable refers to a single character. We can reassign an existing char *pointer to be equal to another char * pointer.

char *str = "apple"; // e.g. 0xfff0char *str2 = "apple 2"; // e.g. 0xfe0str = str2; // ok! Both store address 0xfe0

Arrays and PointersWe can also make a pointer equal to an array; it will point to the first element in that array.

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");char *ptr = str;...

Address Value…

0x105 '\0'0x104 'e'0x103 'l'0x102 'p'0x101 'p'0x100 'a'0xf8 0x100

strptr

main()

Arrays and PointersWe can also make a pointer equal to an array; it will point to the first element in that array.

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");char *ptr = str;

// equivalentchar *ptr = &str[0];

// confusingly equivalent, avoidchar *ptr = &str;...

STACKAddress Value

0x105 '\0'0x104 'e'0x103 'l'0x102 'p'0x101 'p'0x100 'a'0xf8 0x100

strptr

main()

String Behavior #6: Adding an offset to a C string gives us a substring that many places past the first character.

Pointer ArithmeticWhen we do pointer arithmetic, we are adjusting the pointer by a certain number of places (e.g. characters).

char *str = "apple"; // e.g. 0xff0char *str2 = str + 1; // e.g. 0xff1char *str3 = str + 3; // e.g. 0xff3

printf("%s", str); // appleprintf("%s", str2); // ppleprintf("%s", str3); // le

Address Value

0xff5 '\0'

0xff4 'e'

0xff3 'l'

0xff2 'p'

0xff1 'p'

0xff0 'a'

TEXT SEGMENT

char *

When we use bracket notation with a pointer, we are performing pointer arithmetic and dereferencing:

char *str = "apple"; // e.g. 0xff0

// both of these add three places to str,// and then dereference to get the char there.// E.g. get memory at 0xff3.char thirdLetter = str[3]; // 'l'char thirdLetter = *(str + 3); // 'l'

Address Value

0xff5 '\0'

0xff4 'e'

0xff3 'l'

0xff2 'p'

0xff1 'p'

0xff0 'a'

TEXT SEGMENT

String Behavior #7: If we change characters in a string parameter, these changes will persist outside of the function.

Strings as ParametersWhen we pass a char * string as a parameter, C makes a copy of the address stored in the char * and passes it to the function. This means they both refer to the same memory location.

int main(int argc, char *argv[]) {char *str = "apple";myFunc(str);...

Address Value

0xfff0 0x10

0xff0 0x10

main()

myFunc()

Strings as ParametersWhen we pass a char array as a parameter, C makes a copy of the address of the first array element and passes it (as a char *) to the function.

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");myFunc(str);...

Address Value…

0x105 '\0'0x104 'e'0x103 'l'0x102 'p'0x101 'p'0x100 'a'

……

0xf 0x100…

main()

myFunc()

Strings as ParametersWhen we pass a char array as a parameter, C makes a copy of the address of the first array element and passes it (as a char *) to the function.

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");// equivalentchar *strAlt = str;myFunc(strAlt);...

Address Value…

0x105 '\0'0x104 'e'0x103 'l'0x102 'p'0x101 'p'0x100 'a'

……

0xf 0x100…

main()

myFunc()

Strings as ParametersThis means if we modify characters in myFunc, the changes will persist back in main!

void myFunc(char *myStr) {myStr[4] = 'y';

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");myFunc(str);printf("%s", str); // apply...

Address Value…

0x105 '\0'0x104 'e'0x103 'l'0x102 'p'0x101 'p'0x100 'a'

……

0xf 0x100…

main()

myFunc()

Strings as ParametersThis means if we modify characters in myFunc, the changes will persist back in main!

void myFunc(char *myStr) {myStr[4] = 'y';

int main(int argc, char *argv[]) {char str[6];strcpy(str, "apple");myFunc(str);printf("%s", str); // apply...

Address Value…

0x105 '\0'0x104 'y'0x103 'l'0x102 'p'0x101 'p'0x100 'a'

……

0xf 0x100…

main()

myFunc()

Strings In Memory1. If we create a string as a char[], we can modify its characters because its memory

lives in our stack space. 2. We cannot set a char[] equal to another value, because it is not a pointer; it refers

to the block of memory reserved for the original array.3. If we pass a char[] as a parameter, set something equal to it, or perform arithmetic

with it, it’s automatically converted to a char *.4. If we create a new string with new characters as a char *, we cannot modify its

characters because its memory lives in the data segment.5. We can set a char * equal to another value, because it is a reassign-able pointer.6. Adding an offset to a C string gives us a substring that many places past the first

character.7. If we change characters in a string parameter, these changes will persist outside of the

function.

Recap• Searching in Strings• Practice: Password Verification• Demo: Buffer Overflow and Valgrind• Pointers• Strings in Memory

Next time: Arrays and Pointers

Extra Practice

char* vs char[] exercisesSuppose we use avariable str as follows:

For each of thefollowing instantiations:

• Will there be a compile error/segfault?• If no errors, what is printed?

str = str + 1;str[1] = 'u';printf("%s", str)

1. char str[7];strcpy(str, "Hello1");

2. char *str = "Hello2";

3. char arr[7];strcpy(arr, "Hello3");char *str = arr;

4. char *ptr = "Hello4";char *str = ptr;

char* vs char[] exercisesSuppose we use avariable str as follows:

For each of thefollowing instantiations:

• Will there be a compile error/segfault?• If no errors, what is printed?

str = str + 1;str[1] = 'u';printf("%s", str)

1. char str[7];strcpy(str, "Hello1");

2. char *str = "Hello2";

3. char arr[7];strcpy(arr, "Hello3");char *str = arr;

4. char *ptr = "Hello4";char *str = ptr;

Compile error (cannot reassign array)Segmentation fault (string literal)

Prints eulo3 Segmentation fault (string literal)

CS107, Lecture 5 - Stanford University · CS107, Lecture 5 More C Strings Reading: K&R (1.6, 5.5,...

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